The invention relates to an amplifier circuit for audio-frequency signals. An amplifier circuit of this type is known from U.S. Pat. No. 3,595,998 A.
Usually, high-resistive resistances are used for setting the operating point at high-resistance amplifier inputs, and for bias-voltage coupling to capacitive signal sources. FIG. 1 illustrates a corresponding circuit arrangement in accordance with the prior art. Here, C1 represents a capacitive signal source of 50 pF in the form of a capacitor-microphone capsule, which is coupled to a DC supply Vbias1 of +60 V via a high-resistive resistance R1 of 3 GOhms. The useful signal S1 of the capacitive signal source C1, for example an audio signal, is supplied to the high-resistance, non-inverting input (+) of an amplifier IC1 via a series-coupling capacitance C2 of 1 nF (which is inserted into the signal path for separating the operating-point voltages). The output signal S2 of the amplifier IC1 is fed back to the inverting input (xe2x88x92) of the amplifier IC1 in the manner of negative feedback. In terms of the signal voltage, this results in an amplification of V=1, making available the output signal S2 with a low source impedance, which carries the same useful-signal information as the signal S1 with respect to value and phase. A bias source Vbias2 of +5 V, which is coupled to the non-inverting input (+) of the amplifier IC1 via a high-resistance resistance R2 of 3 GOhms, is provided for setting the operating point of the amplifier IC1.
A circuit arrangement similar to the one in FIG. 1 is known from U.S. Pat. No. 3,595,998 A. This known amplifier circuit like wise has a capacitor-microphone capsule M as the capacitive voltage source; its operating point is determined by an ohmic resistance RV, which is connected to a first bias-voltage source UP. To effect a decoupling between the first bias-voltage source UP and a second bias-voltage source UB in order to set the operating point of the downstream amplifier, a coupling capacitance CK is disposed in the signal path between the capacitor-microphone capsule M and the gate electrode 3 of the amplifier FET. The operating point of the amplifier transistor is determined by a resistance-divider network comprising the ohmic resistances Rj, R2, R3, RV and a diode D. Power-supply voltages of arbitrary polarity can be used for the bias-voltage source UB, because field-effect transistors generally have a symmetrical construction, so the source and drain electrodes exchange functions depending on the applied voltagexe2x80x94that is, the respective electrode having the more negative voltage (in an N-channel model) assumes the role of the source. The output signal is obtained symmetrically in the same manner at the source and drain electrodes by the components R4, R5, C and Tr. Only the operating voltage at the gate electrode must be adapted as a function of the polarity of the power-supply voltage, because the operating voltage does not generally correspond to one-half the power-supply voltage. This is effected by the diode D in series with the resistance R2. In the event of a negative supply voltage, the operating point for the amplifier FET is produced by the voltage divider R1/R3. In this instance, the diode D is blocked and ineffective. In the case of a positive power-supply voltage, an inverted-voltage-divider ratio is necessary. This is accomplished by making the diode D conductive and connecting the resistance R2 in parallel to the resistance R3.
The minimal value of the coupling resistances R1, R2 or RV that is theoretically necessary results from the desired lower limit frequency of the useful signal S1 to be transmitted. For example, with a lower limit frequency of 20 Hz and a signal-source capacitance of 50 pF, the resulting value of the coupling resistances R1, R2 or RV, which operate in parallel with respect to the load of the signal source C1, or RV (and whose parallel switching is effective as a load of the signal source) would be 160 MOhms. This type of resistance value generates a very high noise voltage, which is, however, reduced to the ratio of the resistance value of the parallel circuit comprising the two coupling resistances R1, R2 or RV to the value of the impedance of the capacitive signal source, corresponding to the voltage division. It is also the case that, when the resistance value of the parallel circuit comprising the resistances R1, R2 or RV is increased by a specific factor, the noise voltage is further reduced by this factor; in contrast, the noise voltage generated in the parallel resistances R1, R2 or RV only increases by the root of the named factor, in accordance with known laws of physics. With respect to calculations, this means that a noise gain of 3 dB is attained with each doubling of the resistance value.
Unfortunately, this increase in resistance is associated with a considerable drawback: The time that passes from the switching of the operating voltages Vbias 1 and Vbias 2 (switch-on of device), or the switching of the bias voltage Vbias1 to the capacitive signal source for loading the source capacitance and the necessary coupling capacitance, also increases linearly. It is common practice to use resistance values of 1 to 3 GOhms. In conventional microphones, the resulting load times or idle times are in a range of 10 to 15 seconds; in microphones having analog-digital conversion, they can be more than 30 seconds because of increased operating-point requirements. Nevertheless, a further increase in the resistance value with the goal of a noise gain would be desirable, because an extensive overlap by other noise sources does not take place until about 10 to 20 GOhms. It is also to be anticipated that, in the case of a further increase in the resistance value for R1, R2 or RV, in practice the coupled operating-point voltages become increasingly imprecise at the signal source or the non-inverting input (+) of the amplifier because of increasingly frequent, unavoidable leakage currents.
It is further known from EP 0 880 225 A2 to feed the output signal of the amplifier back to the connecting point of a series connection comprising a high-resistive series resistance of two antiparallel diodes, the connection being provided for setting the operating point of the amplifier. In the cited reference, this feedback is accomplished by the fact that virtually no differential voltage results at the two ends of the antiparallel diodes (FIGS. 2 through 5), so the detrimental capacitance parallel to the diodes remains ineffective. In the circuit according to EP 0 880 225 A2, the operating point of the signal source is not set by way of a separate bias-voltage source, so no coupling capacitance is present in the signal path between the signal source and the amplifier. In this known circuit, therefore, there is no issue of the shortest possible charging time because of the absent coupling capacitance.
The same can be said for the amplifier circuit according to U.S. Pat. No. 5,589,799 A, in which there is also no biased microphone capsule as a signal source, and thus also no coupling capacitance in the signal path between the signal source and the amplifier.
It is the object of the invention to attain a considerable noise gain in an amplifier circuit of the type mentioned at the outset, without having to allow for the disadvantages of very high idle times and an excessive influence of leakage currents. Advantageous embodiments and modifications of the amplifier circuit according to the invention ensue from the dependent claims.
The invention is based on the consideration of replacing the coupling resistances R1, R2 with a network comprising the series connection of a nonlinear resistance and a high-resistive coupling resistance. The coupling resistance that determines the load times of the source capacitance and the coupling capacitance C2 can have relatively small dimensions, because the nonlinear resistance is conductive with a low impedance during the load times, and after the charging of the source capacitance and the coupling capacitance, the resistance automatically assumes a high resistance value that is necessary for improving the noise performance.